Kroto et al., Nature, 318, 162 (1985) were among the first to prepare and characterize a highly stable, spherical "soccerball" shaped allotrope of carbon containing 60 carbon atoms. This allotrope, and other, higher spherical allotropes of carbon containing 70 or more carbon atoms, subsequently named fullerenes, have since been isolated by several groups of workers from carbonaceous soot produced by evaporating carbon, usually graphite, in an inert gas atmosphere, and allowing the vapor to condense on a surface. WO 92/04279 and Kratschmer et al., Nature, 347,, 354 (1990) disclose a preparative method wherein graphite rod electrodes are vaporized by application of high intensity electric current in an inert atmosphere such as helium at a pressure of about 50-400 torr, preferably about 100 torr. The resulting black soot is scraped from the collecting surfaces inside the evaporation chamber and dispersed in a solvent such as benzene. The C.sub.60 and C.sub.70 fullerenes are soluble and readily separated from the residual, insoluble soot. "Yields" of fullerenes, i.e. the percentage of fullerenes in the deposited soot, of 2 to 10% are disclosed.
Haufler et al., Mail. Res. Soc. Symp. Proc. 206, 627 (1991), disclose an apparatus and method of preparing soot containing 10% to 15% fullerenes, by vaporizing graphite in an AC carbon arc struck between two coaxially opposed graphite electrodes separated by a gap of &lt;1 mm to 5 mm, in a flow of helium directed at the arc zone at right angles to the electrodes. A carbon evaporation rate of 1 g per min from a 6 mm diameter graphite rod and a fullerene production rate of up to 10 g per hour (15% yield) was reportedly obtained at a helium pressure of 100-200 torr. A similar apparatus and method is described by Curl and Smalley, Sci. Amer. [10], 54 (1991). In both the Curl and Haufler methods, helium flows to the evaporation chamber from an inlet located at right angles to the axis of the arcing, coaxial electrodes.
Haufler et al., ibid, page 633-634, also describe an alternative fullerene production apparatus wherein a rotating graphite sample is subjected to heat and Q-switched laser activation using a very slow flow of helium directed coaxially over the sample. A 10% yield of fullerenes, based on carbon evaporated, was obtained at 1200.degree. C.; higher yields were predicted from operation at higher temperatures but were not demonstrated.
Whetten et al., Matl. Res. Soc. Symp. Proc. 206, 639 (1991), disclose fullerene production in "yields" said to be considerably higher than reported by others, using an AC carbon arc method similar to that of Kratschmer et al. operating in helium at 100-350 torr pressure. In an experiment wherein 50% of a 6 inch.times.0.25 inch diameter rod electrode of high purity graphite was evaporated, 0.5 g of soot was obtained, 25-40% of which was soluble in boiling toluene. Helium pressure is cited as a major factor determining fullerene yield.
A commercial fullerene production apparatus, "Bucky II", available from Ulvick Industries, Inc., is claimed in Ulvick's literature to provide over 300 g of soot in an 8 hour period. The apparatus employs two rotating/translating graphite rod electrodes of either 0.25 inch (0.635 cm) or 0.5 inch (1.3 cm) diameter, each arcing against a coaxially aligned 1 inch diameter stationary water-cooled electrode having a graphite block attached to its end. The electrodes are located inside a vacuum chamber equipped with a water-cooled chimney wherein soot collects. The chamber is operated in helium at 150-200 torr pressure.
The present invention provides an improved apparatus and process for producing fullerenes by arc evaporation of carbon in a flow of inert gas, the improvement residing in several factors, including the surprising discovery that fullerene yields are highly influenced by the direction of flow of the inert gas.